This invention relates to the treatment or control of plant diseases and is particularly concerned with the treatment or control of fungal and bacterial diseases of the rice plant. It is also concerned with novel compositions for the treatment or control of plant diseases and with novel compounds for use in such compositions.
Rice, one of the most important cereal grains, is grown in coastal plains, tidal deltas, and river basins in temperate, tropical and semi-tropical regions. It is the staple food of a large segment of the world's population, the major part of which depends almost entirely upon rice and lives in the poorer and more thickly populated areas of the rice-growing regions.
Rice, or more correctly, the rice plant, like all field crops, is subject to a variety of diseases, the most serious of which is rice blast, a fungal disease caused by Piricularia oryzae. The disease is prevalent in most of the humid rice-producing regions of the world. The aerial part of the plant is attacked. The most conspicuous symptom is neck rot which is characterized by the necks breaking over. Other symptoms are the blighting or blasting of the heads, spots on the leaves, leaft sheaths and stems. The overall result of the disease is a decrease in yield and quality of the rice. The rice plant is likewise subject to fungal disease (sheath blight) caused by Corticium sasakii as well as bacterial attack (leaft blight) caused by Xanthomonas oryzae.
Control measures appear to have developed along two principal lines, cultural and chemical. The various cultural control measures developed include production of resistant varieties of rice, timing of transplanting, clean cultivation, seed selection and controlled irrigation.
Chemical measures of control such as the use of fungicidal seed dressings and foliage fungicides are prophylactic in nature and have little if any therapeutic value. The agents most commonly used for such treatments are organic mercurials, copper sulfate, benzoquinones, naphthoquinones, thiuram disulfate, dithiocarbamates, pentachlorobenzyl alcohol and O,O-diethyl-S-benzylthiophosphate. More recently, therapeutic measures of control using antibiotics such as blasticidin, kasugamycin and blasticidin-S-benzylamino-benzene sulfonate have come into use.
Such chemical methods of control, however, are not satisfactory for one or more reasons such as a low level of effectiveness, inhibition of seed germination, tendency toward phytotoxic effects, high material costs, and in the case of mercury compounds, the presence of toxic residue on the treated crop. Additionally, the use of blasticidin requires extreme care in its use because of its toxicity.